1. Field of the Invention
The present invention relates generally to an improvement in an exhaust emission purifying apparatus, and in particular, relates to a reducing agent container having an improved structure thereof, which is to be used in an exhaust emission purifying system for reductively purifying nitrogen oxides (NOx) in the exhaust gas using a liquid reducing agent. More particularly, the present invention relates to an improved technique for forming a cap member which seals a reducing agent inlet port of the reducing agent container, to have a lockable structure, to thereby prevent undesirable access to the reducing agent container.
2. Description of the Related Art
A conventional exhaust emission purifying apparatus, as a proposal for an exhaust emission purifying system for eliminating NOx contained in the exhaust gas of an engine, in which a liquid reducing agent of necessary amount according to engine operating conditions is injection-supplied to the exhaust upstream of a reduction catalytic converter disposed in an engine exhaust emission system, so that NOx in the exhaust gas and the liquid reducing agent are subjected to the catalytic-reduction reaction, to thereby purify NOx into harmless components, has been provided (refer to Japanese Unexamined Patent Publication No. 2000-27627, for example). Here, for the reduction reaction, ammonia which has a good reactivity to NOx is used, and as the liquid reducing agent, the urea aqueous solution which is hydrolyzed with the exhaust heat and the water vapor in the exhaust gas to generate ammonia, is used.
However, when foreign substances other than the liquid reducing agent, such as the dust, cigarette butt or dottle and the like, are intentionally dropped into a reducing agent container for the purpose of bothering, such foreign substances pass from the reducing agent container through reducing agent supply piping to enter the inside of a reducing agent supplying device, thereby causing the failure. In order to prevent undesirable access to the reducing agent container, such as mischief, it is absolutely desirable to lock a cap for a reducing agent inlet port by means of a lock, so as to make the cap impossible to be freely removed. However, in the case where a known structure in which a lock is incorporated into a cap center portion, as conventionally shown in a filler cap and the like for a fuel tank, is applied to the cap for the reducing agent container, there is a possibility that the liquid reducing agent in the container intrudes the inside of the lock, and solute (urea) of the liquid reducing agent is crystallized to be accumulated in the lock so that locking and unlocking operations cannot be performed.